Extraction of the synchronisation raises in fact in this case a problem which is not met with when the video signal is composite (SECAM, PAL or NTSC for example) and when the line synchronisation is provided by pulses placed in each blanking interval (line and frame) and using levels forbidden to the luminance signal. Recognition of the signals is then simple.
Synchronisation is also simple when the signal is purely digital and when the data transmitted has a statistical independence which may be intrinsic or obtained artificially by mixing (modulo 2 addition of a pseudo-random sequence). The transitions are then in fact in a number sufficient for retrieval of the clock and for estimating the amplitude and the average value of the signal, so adjusting the decoding threshold (or thresholds) of the data and, consequently, retrieving the synchronisation patterns.
It is not at all the same in the case of a base band time multiplex comprising an image signal which, because it is redundant by nature and has a very variable mean value, breaks the statistical independence and prohibits the use of fixed thresholds chosen once and for all with respect to the original signal.
The problem is particularly acute in the case of a multiplex where the extreme levels of the data signal do not exceed the analog level, which makes it impossible to detect the digital signals and in which the analog signals have considerable power at the clock frequency of the digital signals. This latter situation is met with more especially when the television multiplex associates a duobinary coded data signal with component analog multiplexing (or MAC). The flowrate of the binary elements of the digital signal is 10.125 Mbits/s and corresponds to a spectral band in which the time compressed analog signal has appreciable energy.
The field of application contemplated by the invention concerns the system known under the name of "D2-MAC-PAQUETS". This system, which associates in the same base band signal a MAC type signal and data at 10.125 Mb/s coded in duobinary form is derived from the C-MAC/PACKET system proposed by F.R.U. for satellite broadcasting. However, the D2-MAC-PACKET system applies also to broadcasting over land carriers not offering the bandwidth B =27 MHz required for direct satellite television in the 12 GHz band. It allows in particular broadcasting in 7 or 8 MHz channels using vestigial side band amplitude modulation.